Lens barrel and imaging device

ABSTRACT

A lens barrel includes: an electromechanical conversion element; an elastic body having a joining surface, a drive surface, and grooves; a motion member rotating by vibration wave of the drive surface; a rotating ring having a recess part and rotating by rotation of the motion member; a moving ring engaged to the recess part and moving to an optical axis direction by rotation of the rotating ring; and a lens held in the moving ring; wherein the element is made of a material having sodium potassium niobate, potassium niobate, sodium niobate or barium titanate, wherein a value of T/(B+C) is within a range of from 1.3 to 2.8 when: length from the drive surface to a base unit of the groove is defined as T; length from the base unit of the groove to the joining surface is defined as B; and thickness of the element is defined as C.

CLAIM OF PRIORITY

This is a Continuation of application Ser. No. 16/314,628 filed Dec. 31,2018, which is a National Stage Application of PCT/JP2017/024209 filedJun. 30, 2017, which in turn claims priority from Japanese patentapplication JP 2016-130661 filed on Jun. 30, 2016, the contents of whichare hereby incorporated by reference into this application.

BACKGROUND

The present invention relates to a lens barrel and an imaging device.

A vibration wave motor generates a progressive vibration wave(hereinafter referred to as a progressive wave) on a drive surface of anelastic body through making use of expansion/contraction of apiezoelectric body (see Japanese Patent Examined Publication H1-17354).A vibrator in such a vibration wave motor is generally configured of anelectromechanical conversion element (hereinafter referred to as apiezoelectric body) and an elastic body. Up until now, piezoelectricbodies have been made of a material called lead zirconate titanate. Thismaterial is commonly referred to as “PZT”. However, in recent years, alead-free material and the application of this material to a vibrationwave motor have been studied in order to lessen burden on theenvironment (see Japanese Patent Examined Publication H1-17354).

SUMMARY

The present invention is a lens barrel comprising: an electromechanicalconversion element; an elastic body which has a joining surface on whichthe electromechanical conversion element is joined and a drive surfaceon which a vibration wave is generated due to vibration of theelectromechanical conversion element, the elastic body on which aplurality of grooves are formed; a motion member which makes contactwith the drive surface and is configured to rotate by the vibrationwave; a rotating ring which has a recess part and is configured torotate by rotation of the motion member; a moving ring which has aprotrusion part engaged to the recess part and is configured to move toan optical axis direction by rotation of the rotating ring; and a lenswhich is held in the moving ring; wherein the electromechanicalconversion element is made of a material having sodium potassiumniobate, potassium niobate, sodium niobate or barium titanate as aprinciple component, wherein a value of T/(B+C) is within a range offrom 1.3 to 2.8 when: length from the drive surface to a base unit of atleast one groove of the plurality of grooves is defined as T; lengthfrom the base unit of the groove to the joining surface is defined as B;and thickness of the electromechanical conversion element is defined asC.

An imaging device in the present invention comprises the lens barrel anda camera body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic cross-sectional diagram of a lens barrel to which avibration wave motor is incorporated and a camera of the embodiments.

FIG. 2 is a perspective view illustrating part of the vibrator and themover cut out from the vibration wave motor.

FIG. 3 is a diagram for illustrating the piezoelectric body, where (a)illustrates a surface which joins with the elastic body and (b)illustrates the rear of that surface.

FIG. 4 is a block diagram for explaining a drive device of the vibrationwave motor according to the embodiment.

FIG. 5 is a diagram for explaining an equivalent circuit of the vibratorin the vibration wave motor, where (a) illustrates the equivalentcircuit and (b) is an equation for finding a mechanical quality factor.

FIG. 6 is a graph for illustrating the results of calculating the Lmvalue using CAE analysis when changing T value, B value, and C value,respectively.

FIG. 7 is a graph for illustrating the relationship between T/(B+C) anddrive voltage.

FIG. 8 is a diagram for explaining the relationship between T/(B+C) andthe behavior of the protruding unit in the vibrator, where (a)illustrates an example where T/(B+C) is low and (b) illustrates T/(B+C)is high.

FIG. 9 is a diagram of a sequence when the vibration wave motor isstarted up.

FIG. 10 is the graph of FIG. 7 with number values added.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of a vibration wave motor 10 are described in detail belowwith reference the attached drawings. FIG. 1 is schematiccross-sectional diagram of a lens barrel 20 to which a vibration wavemotor 10 is incorporated and a camera 1 of the embodiments.

In the embodiments, the annular vibration wave motor 10 is given as anexample of a vibration wave motor.

A lens barrel 20 includes an outer fixed tube 31 and an inner fixed tube32. A motor unit that includes the vibration wave motor 10 is fixedbetween the outer fixed tube 31 and the inner fixed tube 32.

A drive circuit 40 is provided between the outer fixed tube 31 and theinner fixed tube 32 of the lens barrel 20 and is configured to drive andcontrol the vibration wave motor 10, detect the number of revolutions ofthe vibration wave motor 10, detect a vibration sensor, and the like.

Next, the vibration wave motor 10 is described. The vibration wave motor10 includes a vibrator 11 and a mover 15. FIG. 2 is a perspective viewillustrating part of the vibrator 11 and the mover 15 cut out from thevibration wave motor 10.

The vibrator 11 is configured of an electromechanical conversion element(hereinafter referred to as a piezoelectric body 13) exemplified by apiezoelectric element or an electrostrictive element that convertselectrical energy into mechanical energy, and an elastic body 12 joinedto the piezoelectric body 13. Progressive waves are generated in thevibrator 11. In this embodiment, nine progressive waves are described asexamples.

The elastic body 12 is made of a metal material having a high sharpnessof resonance. The elastic body 12 has an annular shape. Grooves 12 c arecut into a surface of the elastic body 12 that is opposite to thesurface joined to the piezoelectric body 13. A protruding unit 12 b(portion with no grooves 12 c) of the elastic body 12 has a tip surfacethat is a drive surface 12 a and is brought into pressure contact withthe mover 15. The grooves 12 c are cut out in order to allow the elasticbody 12 to approach the piezoelectric body 13 side as closely aspossible to a neutral plane of the progressive wave and amplify theamplitude of the progressive wave on the drive surface 12 a.

An inner peripheral side of the elastic body 12 is provided with aflange unit 12 e that extends in a radial direction. The flange unit 12e is formed with a notch unit (not shown). A protruding unit (not shown)formed on a fixing unit 14 is fitted into the notch unit to restrictmovement of the elastic body 12 in a circumferential direction.

FIG. 3 is a diagram for illustrating the piezoelectric body 13, whereFIG. 3(a) illustrates a surface which joins with the elastic body andFIG. 3(b) illustrates the rear of that surface. The joining surface ofthe piezoelectric body 13 is divided into two phases (an A-phase and aB-phase) along the circumferential direction. In each phase, elementswith alternating polarities every half wavelength are arranged in a row,and a quarter wavelength interval is provided between the A-phase andthe B-phase. Electrodes that are provided on a front surface andelectrode patterns are described later.

The mover 15 is made of a lightweight metal such as aluminum. A slidingsurface 15 a of the mover 15 is provided with a sliding material on afront surface thereof to improve wear resistance. A vibration absorbingmember 23 such as rubber is disposed on a side of the mover 15 oppositeto the vibrator 11. This member is used to absorb vibration of the mover15 in a vertical direction. An output transmission member 24 is disposedon the vibration absorbing member 23.

The vibration absorbing member 23 such as rubber is disposed on themover 15 to absorb vibration of the mover 15 in the vertical direction.The output transmission member 24 is disposed on the vibration absorbingmember 23.

The output transmission member 24 restricts movement of the mover 15 ina pressure direction and the radial direction using a bearing 25provided on the fixing unit 14. The output transmission member 24 has aprotruding unit 24 a from which a fork 35 connected to a cam ring 36 isfitted. The cam ring 36 rotates together with rotation of the outputtransmission member 24.

In the cam ring 36, a key groove 37 is cut obliquely to the cam ring 36.A fixing pin 38 provided on an AF ring 34 is fitted into the key groove37. Rotationally driving of the cam ring 36 allows the AF ring 34 todrive in a straight-ahead direction in an optical axis direction andstop the AF ring 34 at a desired position.

A vibration transmission prevention member 16 such as a nonwoven fabricor felt is provided between the piezoelectric body 13 and a pressurizingspring 18. The vibration transmission prevention member 16 is configuredto not transmit vibration of the vibrator 11 to the pressurizing spring18, a press ring 19, or any other component.

The pressurizing spring 18 is configured of a disc spring or a wavewasher.

The press ring 19 is mounted to the fixing unit 14 using a spring.Through fixing the press ring 19 to the fixing unit 14, the outputtransmission member 24, the mover 15, the vibrator 11 and thepressurizing spring 18 are configured as one motor unit.

As described above, FIG. 3 is a diagram illustrating the piezoelectricbody 13, where FIG. 3(a) illustrates a surface which joins with theelastic body and FIG. 3(b) illustrates the rear of that surface. Aplurality of electrode units 131 are formed on a first surface 13A,which is the joining surface of the piezoelectric body 13. In thisembodiment, 16 electrode units 131 are provided. These electrode units131 each have a length corresponding to half the wavelength of theprogressive wave along the circumferential direction.

The electrode units 131 are divided into groups of eight on each side.One group is configured to transmit A-phase drive voltage and the othergroup is configured to transmit B-phase drive voltage. Electrode units131C corresponding to a quarter wavelength and electrodes 131Dcorresponding to a three-quarter wavelength are provided between theA-phase and the B-phase for a total of 18 electrode units 131.

In this embodiment, the side provided with the 18 electrode units 131joins with the elastic body 12.

As illustrated in FIG. 3(b), a second side 13B opposite to the firstside 13A is provided with an electrode at a position on the firstsurface 13A provided with the A-phase electrode unit. This electrode isformed by joining the group of A-phase electrodes.

Similarly, electrodes formed by joining the group of B-phase electrodesare provided at a position of the group of B-phase electrodes. Quarterwavelength electrode units 132C and three-quarter wavelength electrodeunits 132D are provided between the A-phase and the B-phase to form atotal of four electrode units 132.

In this embodiment, an A-phase drive signal and a B-phase drive signalare applied to the second surface, and a quarter wave length unitshort-circuits with the elastic body 12 due to conductive paint to begrounded.

The material of the piezoelectric body 13 used in this embodiment iscalled a lead-free material and has sodium potassium niobate, potassiumniobate, sodium niobate or barium titanate as a principle component.

FIG. 4 is a block diagram for explaining a drive device 80 of thevibration wave motor 10 according to the embodiment.

First, a control unit 68 and drive of the vibration wave motor 10 aredescribed.

An oscillator 60 generates a drive signal at a predetermined frequencybased on a command from the control unit 68. A phase shifting unit 62divides the drive signal generated by the oscillator 60 into two drivesignals having different phases.

An amplifier 64 boosts each of the two drive signals separated by thephase shifting unit 62 to a predetermined voltage. The drive signalsoutput from the amplifier 64 are transmitted to the vibration wave motor10. Applying these drive signals to the vibration wave motor 10 allowsthe vibrating body to generate a progressive wave and drive the mover15.

A rotation detector 66 is configured of, for example, an optical encoderor a magnetic encoder. The rotation detector 66 detects position orspeed of a driven object driven by drive of the mover 15 and transmitsthe detected value to the control unit 68 as an electrical signal.

The control unit 68 controls drive of the vibration wave motor 10 basedon a drive command output from a CPU in the lens barrel 20 or in thecamera 1 body. The control unit 68 receives a detection signal from therotation detector 66, determines position information and speedinformation on the basis of the value of the detection signal andcontrols frequency of the oscillator 60 so as to position the vibrationmotor 10 at a target position. The control unit 68 changes the phasedifference of the phase shifting unit 62 when the direction of rotationis switched.

Next, operation of the vibration wave motor 10 according to thisembodiment is described.

When a drive command is issued from the control unit 68, the oscillator60 generates a drive signal. This drive signal is divided by the phaseshifting unit 62 into two drive signals having different phases by 90°and boosted to a predetermined voltage by the amplifier 64.

The drive signals are applied to the piezoelectric body 13 of thevibration wave motor 10 to excite the piezoelectric body 13. Thisexcitation causes ninth-order bending vibration to be generated in theelastic body 12.

The piezoelectric body 13 is divided into the A-phase and the B-phaseand the drive signals are applied to the A-phase and the B-phase,respectively.

The positional phases of the ninth-order bending vibration generatedfrom the A-phase and the ninth-order bending vibration generated fromthe B-phase are shifted from each other by a quarter wavelength. Inaddition, the A-phase drive signal and the B-phase drive signal areshifted from each other by 90°. Therefore, the two bending vibrationsare combined to form nine progressive waves.

Elliptical motion occurs at the crest of the progressive wave.Therefore, the mover 15 which has made pressure contact with the drivesurface 12 a is frictionally driven by this elliptical motion. Anoptical encoder is disposed in the driven body driven by drive of themover 15. This optical encoder outputs an electrical pulse which istransmitted to the control unit 68. The control unit 68 can acquirecurrent position and current speed on the basis of this the electricalpulse.

In this embodiment, as described above, a lead-free material is used forthe piezoelectric body 13 in order to lessen burden on the environment.

However, the inventors of the present invention conducted extensivestudy and found that it is difficult to obtain driving performancesimilar to that of a PZT (lead zirconate titanate) piezoelectric bodyunder the same conditions when the vibration wave motor 10 is equippedwith the lead-free piezoelectric body 13.

In order to study the reason behind this, the inventors used computeraided engineering (CAE) analysis and other methods to investigate andfound that the lead-free piezoelectric body 13 and PZT have differentdensities.

When a niobium-based material is used for the lead-free piezoelectricbody 13, the lead-free piezoelectric body 13 has a density of 4.2 to4.7×10³ kg/m³ and when a barium titanate-based material is used for thelead-free piezoelectric body 13, the lead-free piezoelectric body 13 hasa density of 5.5 to 6.0×10³ kg/m³. In contrast, PZT has a density of 7.7to 7.8×10³ kg/m³.

In other words, the lead-free piezoelectric body 13 is 20-plus % to40-plus % less dense than PZT.

(Equivalent Circuit)

FIG. 5 is a diagram for explaining an equivalent circuit of the vibrator11 in the vibration wave motor 10, where FIG. 5(a) illustrates theequivalent circuit and FIG. 5(b) is an equation for finding a mechanicalquality factor Qm.

In the diagrams, Lm represents equivalent inductance, Cm representsequivalent capacitance, R represents resonance resistance and Cdrepresents electrostatic capacity of the piezoelectric body 13.

The values of Lm and Cm have an effect on the resonance characteristicsof the vibrator 11. The mechanical quality factor Qm is a scaleindicating resonance characteristics, with a larger Qm value indicatingbetter resonance characteristics.

As can be seen from Equation 1, the value of Qm increases as the Lmvalue increases.

Table 1 below shows the Lm values and Cm values calculated using CAEanalysis when various materials were used for the piezoelectric body 13.

Here, a model with the following dimensions was used as thepiezoelectric body 13:

Outer diameter: 62 mm

Inner diameter: 55 mm

Thickness of vibrator 11: 4.22 mm

Number of grooves 12 c provided on drive surface 12 a side: 48

Depth of groove 12 c: 1.92 mm.

TABLE 1 Various factor values of Density equivalent circuit 10³ kg/m³ Lm[H] Cm [nF] PZT 7.7 0.341 0.064 Barium titanate 6.0 0.325 0.065Niobium-based 4.2 0.313 0.066 material

As shown in Table 1, the Lm value with PZT is 0.341, whereas the Lmvalue with a barium titanate-based material is 0.325 and the Lm valuewith a niobium-based material is 0.313. In other words, the Lm valuedecreases as density decreases. When a lead-free piezoelectric body 13is incorporated into the vibrator 11, the Lm value is smaller than whena PZT piezoelectric body is incorporated into the vibrator 11. In otherwords, the mechanical quality factor Qm when incorporating a lead-freepiezoelectric body 13 is smaller than when incorporating a PZTpiezoelectric body. Because of this, the inventors found that it isdifficult to achieve desired resonance characteristics whenincorporating a lead-free piezoelectric body 13, compared to whenincorporating a PZT piezoelectric body 13.

Because the vibration wave motor 10 operates under the principle ofusing resonance, drive performance when combined with the mover 15 isdifficult to achieve when desired vibration characteristics in thevibrator 11 cannot be obtained. Therefore, with the vibrator 11employing a lead-free piezoelectric body 13, there is a tendency that itis difficult to obtain desired driving performance.

As a result, the inventors studied trends of the dimensions of vibrators11 with which the Lm value increases in order to improve the resonancecharacteristics of the vibrator 11 employing a lead-free piezoelectricbody 13.

Here, T represents the depth of the grooves 12 c formed in comb teeth ofthe elastic body 12, B represents the thickness to a joining surfacewith the piezoelectric body 13 from the base unit of the groove 12 c,and C represents the thickness of the piezoelectric body 13.

FIG. 6 is a graph for illustrating the results of calculating the Lmvalue using CAE analysis when changing the above-mentioned values withinthe following ranges:

T value: 1.9 to 2.8,B value: 1.3 to 1.8,C value: 0.25 to 0.5.

The results of this calculation show that there is a correlation betweenthe value of T/(B+C) and Lm. This is because Lm increases as the T valueincreases, and also increases as the B value or the C value decreases.

As a result, CAE was used to calculate the Lm value of the vibrator 11in each piezoelectric body material being either PZT or a niobate-basedmaterial with a density lower than PZT (4.2 to 4.7×10³ kg/m³) when thevalue of T/(B+C) was changed. The results of this calculation whenT/(B+C) is 1.2, 1.3, 2 and 2.8 are shown in Table 2.

TABLE 2 Density 10³ T/(B + C) kg/m³ 1.2 1.3 2 2.8 PZT 7.7 Lm = 0.431Niobium- 4.7 Lm = Lm = Lm = Lm = based 0.398 0.414 0.526 0.745 materialPotassium 4.2 Lm = Lm = Lm = Lm = sodium 0.392 0.409 0.522 0.736

Table 2 shows the results of calculating the Lm value using CAE analysiswhen changing the following values within the following ranges:

T value: 1.9 to 3.5,B value: 1.0 to 2.2,C value: 0.25 to 0.8.

The range of 4.2 to 4.7×10³ kg/m³ is the range of density of a generalniobium-based voltage material. Therefore, CAE analysis was performedwith the upper limit density value and lower limit density value of thatrange.

As shown in Table 2, it was found that the Lm value decreases as densitydecreases and that increasing the value of T/(B+C) makes it possible toobtain values that correspond to the vibrator 11 equipped with PZT.

However, damage when the value of T/(B+C) increases is also to beconsidered. Therefore, the inventors tested vibration when equipped witha niobium-based material to investigate resonance characteristics of thematerial as a vibration motor.

The inventors created 12 types of samples in which the piezoelectricbody 13 was made of a material with potassium sodium niobate as aprimary component, the elastic body 12 was made of stainless steel, andthe T and B values of the elastic body 12 and the C value of thepiezoelectric body 13 were changed, and investigated the voltage (drivevoltage) of the drive signal at which each sample could be driven.

The samples had values in the following ranges:

T value: 1.5 to 2.0

B value: 0.35 to 0.75

C value: 0.25 to 0.5.

In addition, the density of the piezoelectric body 13 in the samples was4.4×10³ kg/m³.

It is believed that resonance characteristics in an actual vibratingmotor become better as the drive voltage at which the motor can bedriven decreases, and that resonance characteristics in an actualvibrating motor become worse as the drive voltage at which the motor canbe driven increases.

The results of measurement are shown in FIG. 7.

When the value of T/(B+C) was 1.2, the vibration wave motor 10 did notstart up even after application of a 100 V drive voltage.

The vibration wave motor 10 could be driven at an appropriate drivevoltage at 100 V or less when the value of T/(B+C) was in the range offrom 1.3 to 2.8.

When the value of T/(B+C) was 3.33, the vibration wave motor 10 wasdriven but the rotation state of the mover 15 was slightly unstable.

When the value of T/(B+C) was increased, the Lm value and the Qm valueof the vibrator 11 increased. However, there were some cases where anelectromechanical joining factor Kvn of the vibrator 11 decreased tocause an adverse effect that decreased the efficiency of convertingelectrical energy into mechanical energy.

When the value of T/(B+C) was 3.33, it was believed that theabove-mentioned adverse effect causes the rotation state of the mover 15to become slightly unstable.

First Embodiment

A first embodiment of the present invention has the followingconfiguration based on the results of the above-described tests.

The piezoelectric body 13 is made of a material which has potassiumsodium niobate as a primary component and has a density of 4.2 to4.7×10³ kg/m³. Stainless steel is used for the elastic body 12. Thevalue of T/(B+C) is within a range of from 1.3 to 2.8.

This configuration ensures resonance characteristics as the vibrator 11even if the density of the piezoelectric body 13 decreases and driveperformance when combined with the mover 15.

When T/(B+C) was 1.3, the results of calculation with CAE analysisshowed that the Lm value of the vibrator 11 was about 0.41 and, whenT/(B+C) was 2.8, the Lm value of the vibrator 11 was about 0.74.

Second Embodiment

Next, a second embodiment is described.

In the second embodiment, the piezoelectric body 13 is made of amaterial having barium titanate as a primary component and has a densityof 5.5 to 6.0×10³ kg/m³. Stainless steel is used for the elastic body12. The value of T/(B+C) is in a range of from 1.3 to 2.8.

Even when the piezoelectric body 13 is made of barium titanate, thepiezoelectric body 13 is less dense than PZT, and hence the Lm value ofthe vibrator 11 decreases and sufficient resonance characteristics asthe vibrator 11 cannot be obtained. Under this state, drive performancecannot be obtained even when combined with the mover 15.

Therefore, in this embodiment, the Lm value of the vibrator 11 wascalculated using CAE analysis after changing the value of T/(B+C).

Table 3 shows results when the Lm values of each vibrator 11 werecalculated when the density was 5.5×10³ kg/m³ and 6.0×10³ kg/m³, and thevalue of T/(B+C) has been changed to 1.2, 1.3, 2 and 2.8.

The range of 5.5 to 6.0×10³ kg/m³ is the range of density of a generalbarium titanate-based voltage material, and hence CAE analysis wasperformed with the upper limit density value and lower limit densityvalue of that range. Note that CAE analysis was performed with a T valueof from 1.9 to 3.5, a B value of from 1.0 to 2.2 and a C value of from0.25 to 0.8.

TABLE 3 Density 10³ T/(B + C) kg/m³ 1.2 1.3 2 2.8 PZT 7.7 Lm = 0.431Barium 6.0 Lm = Lm = Lm = Lm = titanate- 0.407 0.424 0.541 0.751 based5.5 Lm = Lm = Lm = Lm = material 0.400 0.418 0.533 0.747

When the value of T/(B+C) was 1.3, the Lm value was about 0.42. When theT/(B+C) was 2.8, the Lm value of the vibrator 11 was about 0.75.

Even when the piezoelectric body 13 was made of a material having adensity of 5.5 to 6.0×10³ kg/m³, the relationship between the value ofT/(B+C) and the Lm was almost the same value as that of material with adensity of from 4.2 to 4.7×10³ kg/m³.

Therefore, the value of T/(B+C) is believed to be appropriate within arange of from 1.3 to 2.8.

When the value of T/(B+C) was from 1.3 to 2.8, both the Lm value of aniobium-based material (density: 4.2 to 4.7×10³ kg/m³) and the Lm valueof a barium titanate-based material (density: 5.5 to 6.0×10³ kg/m³) hadthe same value relationship, and hence a value of T/(B+C) of from 1.3 to2.8 is considered to be within an appropriate range with a density offrom 4.2 to 6.0×10³ kg/m³.

Third Embodiment

Next, a third embodiment of the present invention is described.

FIG. 8 is a diagram for explaining the relationship between T/(B+C) andthe behavior of the protruding unit 12 b in the vibrator 11. FIG. 8(a)illustrates an example where T/(B+C) is low (when, for example, thedepth of the groove 12 c is shallow) and FIG. 8(b) illustrates behaviorof the protruding unit 12 b in the vibrator 11 when T/(B+C) is high(when, for example, the depth of the groove 12 c is deep).

When a progressive vibration wave is generated in the vibrator 11,bending becomes deformed under a state where a thickness portion of anadhesive surface of the piezoelectric body 13 from a base unit of thegroove 12 c in the elastic body 12 and thickness of the piezoelectricbody 13 are matched (in other words, bending deformation occurs whenthere is no protruding unit 12 b).

A neutral plane of the bending exists between the base of the groove 12c in the elastic body 12 and a lower surface of the piezoelectric body13. Swinging motion in the driving direction occurs in the protrudingunit 12 b when adding the bending vibration generated under the statewhere the thickness portion of an adhesive surface of the piezoelectricbody 13 from a base unit of the groove 12 c in the elastic body 12 andthickness of the piezoelectric body 13 are matched.

Motion in the driving direction on a tip (driving unit) of theprotruding unit 12 b is small when T/(B+C) is low and large when T/(B+C)is high.

The magnitude of speed of the protruding unit 12 b is roughly estimatedas follows. Additionally, the motion in the driving direction is definedas ((B+C)/2). The motion in the driving direction occurs under the statewhere the thickness portion of an adhesive surface of the piezoelectricbody 13 from a base of the groove 12 c in the elastic body 12 andthickness of the piezoelectric body 13 are matched. The drive surface 12a moves in the driving direction by (T+(B+C)/2)/((B+C)/2) times. Forexample, when T is increased, swing of the drive surface 12 a increasesby the same amount.

When T/(B+C) is high, the motion in the driving direction is large, andthis increases force applied from the mover 15 to the drive surface 12a. For example, when displacement of the motion in the driving directiondoubles, speed and acceleration also double, and double the force (load)is applied to the drive surface 12 a when the mover 15 that is incontact with the drive surface 12 a tries to move. As a result, thevibration wave motor 10 can fail to drive when, for example, there is alarge speed variation such as that when the vibration wave motor 10starts up.

In the third embodiment, when a lead-free piezoelectric body 13 havingsmall density is incorporated into the vibrator 11, in order to improvethe Lm of the vibrator 11, T/(B+C) is increased to higher than when aPZT piezoelectric body 13 is incorporated into the vibrator 11.Therefore, in this case, the above-described situation is more likely tooccur.

As a result, the inventors conducted an investigation and found that,when the vibration wave motor 10 starts up, the above-mentioned problemcan be solved by increasing the time taken to vary frequency whenfrequency is swept.

FIG. 9 is a diagram of a sequence when the vibration wave motor 10 isstarted up.

Under a state (t0) where there is no drive command from the control unit68, fs0 represents drive frequency, V0 represents drive voltage (=0 V)and the difference between the A-phase and the B-phase is 0°

When a drive command is output from the control unit 68 (t1), the drivefrequency is still set as fs0, the drive voltage is set as a voltage V1and the difference between the A-phase and the B-phase is set as 90°(−90° during inversion driving). At this time, rotational speed is 0.

When drive frequency is gradually decreased and frequency at t2 is f0,the mover 15 is driven.

At a time t4, the frequency becomes “flow” and rotational speed reachesa target speed Rev 1.

In this embodiment, the time taken to vary frequency when frequency isswept is increased according to the value of T/(B+C).

More specifically, the difference in frequency between flow and fs0 andthe difference between the times t4 to t2 are set to be relevant to eachother.

In addition, when the value of T/(B+C) is low, the difference betweenthe times t4 to t2 is shortened and, when the value of T/(B+C) is high,the difference between the times t4 to t2 is lengthened.

Increasing start-up time in this way reduces reaction force from themover 15 applied to the vibrator 11 of the vibration wave motor 10during startup.

When the vibration wave motor 10 is equipped with PZT (T/(B+C): 1.08),the frequency change rate of frequency sweep at startup is about 1 kHz/msec.

When T/(B+C) is within a range of from 1.2 to 1.7, motion of the drivesurface 12 a in the driving direction increases by about 1.1 to 1.4times compared to when T/(B+C) is 1.08. Therefore, the time t4 to t2 isincreased by 1.4 times.

That is, if the frequency change rate of frequency sweep is around 1/1.4and 0.7 kHz/m sec, load on the vibration wave motor 10 is approximatelyequal to or less than that when equipped with PZT (T/(B+C): 1.08).

Further, when T/(B+C) is within a range of from 1.7 to 2.8, motion ofthe drive surface 12 a in the driving direction increases by about 1.4to 2.0 times compared to when T/(B+C) is 1.08.

Therefore, when the time t4 to t2 is increased by 2.0 times, that is,when the frequency change rate of frequency sweep is about 1/2.0 and 0.5kHz/msec, load on the vibration wave motor 10 is approximately equal toor less than that when equipped with PZT (T/(B+C): 1.08).

Varying the frequency change rate according to the T/(B+C) enables thevibration wave motor 10 to start up reliably even if speed fluctuationsuch as that during start up is large (that is, load on the vibrator 11of the vibration wave motor 10 is large).

This embodiment disclosed an example of a vibration wave motor 10 whichused a progressive vibration wave with the number of waves being four ornine. But the same effect can be achieved with other numbers of wavessuch as five to eight, or 10 waves or more, provided that the motor hasthe same configuration and is controlled in a similar manner.

In addition, when T/(B+C) is 1.7, the frequency change rate may be 0.5kHz/msec or less even if the frequency change rate is 0.7 kHz/msec orless.

As illustrated in FIG. 10, in terms of the T/(B+C) explained above inthe first and second embodiments, drive voltage is preferably about 60 Vwhen T/(B+C) is within a range of from 1.76 to 2.8. In addition, thedrive voltage preferably decreases even further when T/(B+C) is within arange of from 1.76 to 2.50.

Note that the embodiments and modified embodiments may be combined asnecessary, but a detailed description thereof is omitted herein.Further, the present invention is not limited by the above-describedembodiments.

What is claimed is:
 1. A lens barrel comprising: an electromechanicalconversion element; an elastic body which has a joining surface on whichthe electromechanical conversion element is joined and a drive surfaceon which a vibration wave is generated due to vibration of theelectromechanical conversion element, the elastic body on which aplurality of grooves are formed; a motion member which makes contactwith the drive surface and is configured to rotate by the vibrationwave; a rotating ring which has a recess part and is configured torotate by rotation of the motion member; a moving ring which has aprotrusion part engaged to the recess part and is configured to move toan optical axis direction by rotation of the rotating ring; and a lenswhich is held in the moving ring; wherein the electromechanicalconversion element is made of a material having sodium potassiumniobate, potassium niobate, sodium niobate or barium titanate as aprinciple component, wherein a value of T/(B+C) is within a range offrom 1.3 to 2.8 when: length from the drive surface to a base unit of atleast one groove of the plurality of grooves is defined as T; lengthfrom the base unit of the groove to the joining surface is defined as B;and thickness of the electromechanical conversion element is defined asC.
 2. The lens barrel according to claim 1, further comprising: acontrol unit configured to control to drive an actuator which has theelectromechanical conversion element, the elastic body and the motionmember.
 3. The lens barrel according to claim 2, further comprising: adetector configured to detect position or speed of the moving ring andtransmit the detected value to the control unit as an electrical signal.4. The lens barrel according to claim 3, wherein the control unit isconfigured to control based on the electrical signal transmitted fromthe detector.
 5. The lens barrel according to claim 4, wherein theactuator is configured to set a voltage to predetermined value when adrive command from the control unit is input.
 6. The lens barrelaccording to claim 5, wherein the actuator is configured to change afrequency after the voltage has been set to the predetermined value. 7.The lens barrel according to claim 6, wherein the actuator is configuredto change a change rate of the frequency based on the value of T/(B+C).8. The lens barrel according to claim 7, wherein the actuator isconfigured to set the change rate of the frequency to 0.7 kHz/msec whenT/(B+C) is within a range of from 1.3 to 1.7, and equal to or less than0.5 kHz/msec when T/(B+C) is within a range of from 1.7 to 2.8.
 9. Animaging device comprising: the lens barrel according to claim 1; and acamera body.